Strain and orientation modulated bandgaps and effective masses of phosphorene nanoribbons

Half metal phase in the zigzag phosphorene nanoribbon

Exploring half-metallic nanostructures is a crucial solution for developing high-performance spintronic devices. Black phosphorene is an emerging two-dimensional material possessing strong anisotropic band structure and high mobility. Based on the first principles calculations, we investigated the electronic and magnetic properties of zigzag phosphorene nanoribbons (ZPNRs) with three different functionalization groups (OH/CN, OH/NO₂, NH₂/NO₂) at the edges. We find that the interplay between edge functionalization and edge oxidation can induce the half metal phase in the ZPNRs, and the half metal phase can be controlled by the external transverse in-plane electric field and the proportion of the functional groups and edge oxidation. The results may pave a new way to construst nanoscale spintronic devices based on black phosphorene nanoribbons.

Tunable electronic structure and magnetic moment in C2N nanoribbons with different edge functionalization atoms

First principles calculations based on density functional theory were carried out to study the electronic and magnetic properties of C₂N nanoribbons (C₂NNRs). The electronic structure could be modified by different methods using saturated or co-saturated H, O, and F on the edges, which can provide a new pathway at the nanoscale for fabricating 2D spintronic materials. It was found that the pristine armchair C₂NNR (A-C₂NNR) is a nonmagnetic semiconductor with a direct band gap, while the pristine zigzag C₂NNRs (Z-C₂NNRs) can show either magnetic semiconductor with an indirect band gap or magnetic metallic behavior depending on its ribbon widths. A-C₂NNRs with one type of atom (H, O or F) saturated on the edges are nonmagnetic, while H and O (F and O) co-saturated A-C₂NNRs show magnetic ground states. H and O (F and O) co-saturated Z-C₂NNRs share a larger magnetic moment compared to the case with H, O and F saturated on the edges. Furthermore, O-saturated Z-C₂NNR is a spin "gapless" semiconductor. Additionally, there is no need to spin flip in the process of electronic transition near the Fermi level. Therefore, C₂NNRs might have potential applications in photoelectronic and spinelectronic devices.

Mechanical stability of a nanotube from monolayer black phosphorus with the [110] direction as the tube's circumference or generatrix

The mechanical properties of black phosphorus (BP) are anisotropic. Correspondingly, the properties of the nanotubes formed by bending the same BP ribbon along different directions are different as well. When bending the ribbon along the [110] direction (i.e., stair direction), or along its perpendicular direction (i.e., ps-direction), s- or ps-BPNT can be obtained. The two types of BPNTs are investigated via molecular dynamics (MD) simulations on their thermal and mechanical properties. The results indicate that, for the thermal stability of the s-BPNTs with similar diameters, s-BPNT is weaker than a-BPNTs (armchair type) but stronger than ps-BPNT, and z-BPNT (zigzag type) is the weakest one. In general, a-BPNT has larger compressive or tensile strength, while s-BPNT and ps-BPNT can bear larger deformation. Under uniaxial compression, s-BPNT has two different breaking patterns at different temperatures. The peculiar properties illustrate the wider application of BPNTs in nanodevices under large deformation.

Strain-tunable electronic and optical properties of BC3 monolayer

Two-dimensional layered nanostructures with unique electronic and optical properties may hold great potential in nanoelectronics and optoelectronics applications. In this work, structural stability, elastic, electronic, and optical properties of BC3 monolayers have been investigated using a first-principles study. The BC3 monolayer can be regarded as a series of hexagonal C rings with the connections of B atoms, which has been tested to be highly dynamically stable. The in-plane stiffness is 316.2 N cm-1, potentially rivalling graphene. A screened hybrid density functional HSE06 is used to calculate the electronic and optical properties. It is found that the BC3 monolayer is an indirect band gap semiconductor with a moderate gap energy of 1.839 eV. Spatial charge distribution to the valence band maximum and the conduction band minimum is analyzed to explore the origin of indirect band gap features. By calculating the complex dielectric function, optical properties considered as excitonic effects are discussed. Besides, the effects of various in-plane strains on electronic and optical properties are explored. Our results of good structural stability, moderate and tunable band gap, and strain-controllable optical properties suggest that the BC3 monolayer holds great promise in the applications of nanoelectronic and optoelectronic devices.

Role of surface adsorption in tuning the properties of black phosphorus

The synergetic effect of O₂and H₂O during the oxidation of black phosphorus (BP) at the atomic level is revealed, and the effects of H₂O and/or O₂on the properties of BP are also investigated.

Unconventional strain-dependent conductance oscillations in pristine phosphorene

We investigated the strain phase diagram of phosphorene and observed strain-tuneable conductance oscillations that are robust against doping and defects.

Modulation of electronic and magnetic properties of edge hydrogenated armchair phosphorene nanoribbons by transition metal adsorption

Based on first-principles calculations, we present a systematic investigation of the electronic and magnetic properties of armchair phosphorene nanoribbons (APNRs) functionalized by 3d transition metal (TM) atoms.

Strain and defect engineering on phase transition of monolayer black phosphorene

Under biaxial strain, SW-2 defect can move inward the phase boundary of α-P and β-P remarkably and promote the phase transition from α-P to β-P, serving as an excellent ‘phase transition catalyzer’.

A first-principles study on zigzag phosphorene nanoribbons passivated by iron-group atoms

We performed a first-principles study on Fe-, Co-, and Ni-terminated zigzag phosphorene nanoribbons (ZPNRs) with different widths. Magnetic edges were observed for Fe- and Co-terminated ZPNRs, whereas Ni-terminated ZPNRs were nonmagnetic. Interestingly, magnetism could be induced in Ni-ZPNRs by external electric fields, and the distribution of the magnetic moments could be tuned by the direction of the electric fields. Furthermore, Fe-ZPNRs and Co-ZPNRs exhibit semi-metallic and metallic characteristics, respectively, whereas Ni-ZPNRs are mainly semiconductors with band gaps generally increasing monotonously with the increase in nanoribbon width. These fascinating properties of iron-group atom terminated ZPNRs indicate their great potential applications in future spintronics, optoelectronics, and information technologies.

Assembling phosphorene flexagons for 2D electron-density-guided nanopatterning and nanofabrication

To build upon the rich structural diversity in the ever-increasing polymorphic phases of two-dimensional phosphorene, we propose different assembly methods (namely, the "bottom-up" and "top-down" approaches) that involve four commonly reported parent phases (i.e. the α-, β-, γ-, and δ-phosphorene) in combination with the lately reported remarkably low-energy one-dimensional defects in α-phosphorene. In doing so, we generate various periodically repeated phosphorene patterns in these so-called phosphorene flexagons and present their local electron density (via simulated scanning tunneling microscopy (STM) images). These interesting electron density patterns seen in the flexagons (mimicking symmetry patterns that one may typically see in a kaleidoscope) may assist as potential 2D templates where electron-density-guided nanopatterning and nanofabrication in complex organized nanoarchitectures are important.

Periodic Arrays of Phosphorene Nanopores as Antidot Lattices with Tunable Properties

A tunable band gap in phosphorene extends its applicability in nanoelectronic and optoelectronic applications. Here, we propose to tune the band gap in phosphorene by patterning antidot lattices, which are periodic arrays of holes or nanopores etched in the material, and by exploiting quantum confinement in the corresponding nanoconstrictions. We fabricated antidot lattices with radii down to 13 nm in few-layer black phosphorus flakes protected by an oxide layer and observed suppression of the in-plane phonon modes relative to the unmodified material via Raman spectroscopy. In contrast to graphene antidots, the Raman peak positions in few-layer BP antidots are unchanged, in agreement with predicted power spectra. We also use DFT calculations to predict the electronic properties of phosphorene antidot lattices and observe a band gap scaling consistent with quantum confinement effects. Deviations are attributed primarily to self-passivating edge morphologies, where each phosphorus atom has the same number of bonds per atom as the pristine material so that no dopants can saturate dangling bonds. Quantum confinement is stronger for the zigzag edge nanoconstrictions between the holes as compared to those with armchair edges, resulting in a roughly bimodal band gap distribution. Interestingly, in two of the antidot structures an unreported self-passivating reconstruction of the zigzag edge endows the systems with a metallic component. The experimental demonstration of antidots and the theoretical results provide motivation to further scale down nanofabrication of antidots in the few-nanometer size regime, where quantum confinement is particularly important.

Quantum confinement in black phosphorus-based nanostructures

The modification of an idealized infinite bulk system by dimensional reduction or structural distortion results in quantum confinement effects (QCEs). For example, dimensional reduction of a black phosphorus structure leads to the realization of few-layer systems, creation of edges and surfaces, nanoribbons, quantum dots, and antidot lattices while structural distortion involves simple bending (including nanotubes) and rippling. Black phosphorus ('phosphorene' in the single-layer limit) has been of recent interest due to its relatively large charge carrier mobility and moderate semiconducting band gap, which remains direct irrespective of the number of layers. In this review the state-of-the-art properties of black phosphorus in its dimensionally reduced and structurally distorted forms are discussed, with emphasis on how quantum confinement impacts the material's properties.

Size and strain tunable band alignment of black-blue phosphorene lateral heterostructures

Single-element lateral heterostructures composed of black and blue phosphorene are not only free from lattice mismatch but also exhibit rich physical properties related to the seamlessly stitched interfaces, providing the building blocks for designing atomically thin devices. Using first-principles calculations, we investigate the influence of interface structure, size effect and strain engineering on the electronic structure, effective masses and band alignment of black-blue phosphorene lateral heterostructures. The lateral heterostructure with an octatomic-ring interface presents a strong metallic feature due to the interface states, while a metal-semiconductor transition takes place in the system with a hexatomic-ring interface upon hydrogen passivation. Following a reciprocal scaling law, the band gap is tuned in a wide energy range by synchronously increasing the widths of black and blue phosphorene or by only widening that of black phosphorene. Moreover, type-II band alignment is observed in the width ranges of 2.0-3.1 nm and 3.7-4.2 nm, out of which it is type-I. However, the band gap and effective masses show small changes if only the width of blue phosphorene is altered. When the lateral heterostructure is tensile loaded, the effective mass ratio of hole to electron is enlarged by an order of magnitude at a strain of 4% along the zigzag direction. Meanwhile, the band alignment undergoes a crossover from type-I to type-II at a strain of 2%, facilitating efficient electron-hole separation for light detection and harvesting.

Graphene-like nanoribbons periodically embedded with four- and eight-membered rings

Embedding non-hexagonal rings into sp²-hybridized carbon networks is considered a promising strategy to enrich the family of low-dimensional graphenic structures. However, non-hexagonal rings are energetically unstable compared to the hexagonal counterparts, making it challenging to embed non-hexagonal rings into carbon-based nanostructures in a controllable manner. Here, we report an on-surface synthesis of graphene-like nanoribbons with periodically embedded four- and eight-membered rings. The scanning tunnelling microscopy and atomic force microscopy study revealed that four- and eight-membered rings are formed between adjacent perylene backbones with a planar configuration. The non-hexagonal rings as a topological modification markedly change the electronic properties of the nanoribbons. The highest occupied and lowest unoccupied ribbon states are mainly distributed around the eight- and four-membered rings, respectively. The realization of graphene-like nanoribbons comprising non-hexagonal rings demonstrates a controllable route to fabricate non-hexagonal rings in nanoribbons and makes it possible to unveil their unique properties induced by non-hexagonal rings.

Graphene nanoribbons consist of carbon atoms arranged in a hexagonal lattice. Despite non-hexagonal rings generally being more unstable, the authors demonstrate the successful synthesis of graphene-like nanoribbons with periodically embedded four- and eight-membered carbon rings, with tailored electronic properties.

Ultra-narrow blue phosphorene nanoribbons for tunable optoelectronics

We report optoelectronic properties of ultra-narrow blue phosphorene nanoribbons (BPNRs) within the state-of-the-art density functional theory framework.

Tunable electronic properties of arsenene/GaS van der Waals heterostructures

On the basis of density functional electronic calculations, we identify that the arsenene/GaS heterostructure is a promising photocatalyst for water splitting and find that its electronic properties can be continuously tuned by external strain.

Elemental two-dimensional nanosheets beyond graphene

The recent progress of elemental two-dimensional nanosheets, beyond graphene, has been summarized with the focus on their preparation and applications.

Arsenene/Ca(OH)2 van der Waals heterostructure: strain tunable electronic and photocatalytic properties

Based on ab initio calculations, we identify that the arsenene/Ca(OH)₂ van der Waals heterostructure is an indirect-band-gap semiconductor and find that its electronic and photocatalytic properties can be continuously tuned by external strain.

Size modulation electronic and optical properties of phosphorene nanoribbons: DFT–BOLS approximation

DFT and BOLS approximations were carried out to study the electronic and optical properties of different sizes of black phosphorus nanoribbons (PNRs) with either zigzag- or armchair-terminated edges.

A theoretical review on electronic, magnetic and optical properties of silicene

Inspired by the success of graphene, various two dimensional (2D) structures in free standing (FS) (hypothetical) form and on different substrates have been proposed recently. Silicene, a silicon counterpart of graphene, is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Since the effective spin-orbit interaction is quite significant compared to graphene, buckling in silicene opens a gap of 1.55 meV at the Dirac point. This band gap can be further tailored by applying in plane stress, an external electric field, chemical functionalization and defects. In this topical theoretical review, we would like to explore the electronic, magnetic and optical properties, including Raman spectroscopy of various important derivatives of monolayer and bilayer silicene (BLS) with different adatoms (doping). The magnetic properties can be tailored by chemical functionalization, such as hydrogenation and introducing vacancy into the pristine planar silicene. Apart from some universal features of optical absorption present in all these 2D materials, the study on reflectivity modulation with doping (Al and P) concentration in silicene has indicated the emergence of some strong peaks having the robust characteristic of a doped reflective surface for both polarizations of the electromagnetic (EM) field. Besides this, attempts will be made to understand the electronic properties of silicene from some simple tight-binding Hamiltonian. We also point out the importance of shape dependence and optical anisotropy properties in silicene nanodisks and establish that a zigzag trigonal possesses the maximum magnetic moment. We also suggest future directions to be explored to make the synthesis of silicene and its various derivatives viable for verification of theoretical predictions. Although this is a fairly new route, the results obtained so far from experimental and theoretical studies in understanding silicene have shown enough significant promising features to open a new direction in the silicon industry, silicon based nano-structures in spintronics and in opto-electronic devices.

Scaling Effect of Phosphorene Nanoribbon - Uncovering the Origin of Asymmetric Current Transport

In this paper, phosphorene nanoribbons (PNRs) are theoretically studied using a multiscale simulation flow from the ab initio level to the tight binding (TB) level. The scaling effects of both armchair PNRs (aPNRs) and zigzag PNRs (zPNRs) from material properties to device properties are explored. The much larger effective mass of holes compared to that of electrons in zPNR is responsible for its asymmetric transport. However, in aPNR, not only the effective mass difference but also the non-equal density of state (DOS) distributions near valence band maximum (VBM) and conduction band minimum (CBM) lead to the asymmetric transport. This non-equal distribution phenomenon is caused by energy band degeneracies near the VBM. Based on these two different mechanisms, PNRs’ asymmetric transport characteristics at the device level are explained, and it is shown that this behaviour can be ameliorated well by reducing the ribbon width in an aPNR MOSFET. Calculation results also indicate that aPNR’s effective mass is comparable to that of a graphene nanoribbon (GNR) at the same bandgap; however, aPNR’s band gap variation is more stable and regular than that of GNR, making it a good candidate for use in low-dimensional nano devices.

Inconsistencies in the Electronic Properties of Phosphorene Nanotubes: New Insights from Large-Scale DFT Calculations

Contrary to recent reports, we show that the electronic properties of phosphorene nanotubes are surprisingly rich and much more complex than previously assumed. We find that all phosphorene nanotubes exhibit an intricate direct-to-indirect band gap transition as the nanotube diameter decreases, a unique property not identified in any prior studies (which claimed either direct or indirect band gaps only) that we uncover with large-scale DFT calculations. We address these previous inconsistencies by detailed analyses of orbital interactions, which reveal that the strain associated with decreasing the nanotube diameter causes a transition from a direct to an indirect band gap for all of the phosphorene nanotubes. We show that our findings are completely general, and extensive calculations across several exchange-correlation functionals verify our conclusions. Most importantly, our results and analyses resolve a long-standing question on the electronic properties of phosphorus nanotubes and bring closure to previously conflicting findings in these unique nanostructures.

Mechanical properties of phosphorene nanotubes: a density functional tight-binding study

Using the density functional tight-binding method, we studied the elastic properties, deformation and failure of armchair (AC) and zigzag (ZZ) phosphorene nanotubes (PNTs) under uniaxial tensile strain. We found that the deformation and failure of PNTs are very much anisotropic. For ZZ PNTs, three deformation phases are recognized: the primary linear elastic phase-which is associated with interactions between neighboring puckers, succeeded by the bond rotation phase-where the puckered configuration of phosphorene is smoothed via bond rotation, and lastly the bond elongation phase-where the P-P bonds are directly stretched up to the maximally allowed limit and failure is initiated by the rupture of the most stretched bonds. For AC PNTs, the applied strain stretches the bonds up to the maximally allowed limit, causing their ultimate failure. For both AC and ZZ PNTs, their failure strain and failure stress are sensitive- while the Young's modulus, flexural rigidity, radial Poisson's ratio and thickness Poisson's ratio are relatively insensitive-to the tube diameter. More specifically, for AC PNTs, the failure strain decreases from 0.40 to 0.25 and the failure stress increases from 13 GPa to 21 GPa when the tube diameter increases from 13.3 Å to 32.8 Å; while for ZZ PNTs, the failure strain decreases from 0.66 to 0.55 and the failure stress increases from 4 GPa to 9 GPa when the tube diameter increases from 13.2 Å to 31.1 Å. The Young's modulus, flexural rigidity, radial and thickness Poisson ratios are 114.2 GPa, 0.019 eV · nm(2), 0.47 and 0.11 for AC PNTs, and 49.2 GPa, 0.071 eV · nm(2), 0.07 and 0.21 for ZZ PNTs, respectively. The present findings provide valuable references for the design and application of PNTs as device elements.

Resolving and Tuning Mechanical Anisotropy in Black Phosphorus via Nanomechanical Multimode Resonance Spectromicroscopy

Black phosphorus (P) has emerged as a layered semiconductor with a unique crystal structure featuring corrugated atomic layers and strong in-plane anisotropy in its physical properties. Here, we demonstrate that the crystal orientation and mechanical anisotropy in free-standing black P thin layers can be precisely determined by spatially resolved multimode nanomechanical resonances. This offers a new means for resolving important crystal orientation and anisotropy in black P device platforms in situ beyond conventional optical and electrical calibration techniques. Furthermore, we show that electrostatic-gating-induced straining can continuously tune the mechanical anisotropic effects on multimode resonances in black P electromechanical devices. Combined with finite element modeling (FEM), we also determine the Young's moduli of multilayer black P to be 116.1 and 46.5 GPa in the zigzag and armchair directions, respectively.

Thermal contact resistance across a linear heterojunction within a hybrid graphene/hexagonal boron nitride sheet

Interfacial thermal conductance plays a vital role in defining the thermal properties of nanostructured materials in which heat transfer is predominantly phonon mediated. In this work, the thermal contact resistance (R) of a linear heterojunction within a hybrid graphene/hexagonal boron nitride (h-BN) sheet is characterized using non-equilibrium molecular dynamics (NEMD) simulations. The effects of system dimension, heat flux direction, temperature and tensile strain on the predicted R values are investigated. The spatiotemporal evolution of thermal energies from the graphene to the h-BN sheet reveals that the main energy carrier in graphene is the flexural phonon (ZA) mode, which also has the most energy transmissions across the interface. The calculated R decreases monotonically from 5.2 × 10(-10) to 2.2 × 10(-10) K m(2) W(-1) with system lengths ranging from 20 to 100 nm. For a 40 nm length hybrid system, the calculated R decreases by 42% from 4.1 × 10(-10) to 2.4 × 10(-10) K m(2) W(-1) when the system temperature increases from 200 K to 600 K. The study of the strain effect shows that the thermal contact resistance R between h-BN and graphene sheets increases with the tensile strain. Detailed phonon density of states (PDOS) is computed to understand the thermal resistance results.

Structural anisotropy results in strain-tunable electronic and optical properties in monolayer GeX and SnX (X = S, Se, Te)

Using first-principles calculations, the structural and electronic properties of group-IV monochalcogenide monolayers are investigated. It is demonstrated that all the monolayers employed here possess moderate indirect bandgaps. In-plane elastic stiffness calculation demonstrates the structural anisotropy in these materials, further resulting in anisotropic response to in-plane strains in their electronic properties and anisotropic optical properties. The bandgaps of GeX and SnX monolayers can be linearly reduced by applied in-plane compressive strains and the semiconductor-to-metal transition can be realized under large compressive strains; while tensile strains exert less influence on the electronic properties in comparison to compressive strains. Some monolayers will experience indirect-to-direct bandgap transition when subjected to proper strains. A further insight into the variation of bandgaps of these monolayers can be obtained from the changing band edges.

Acute mechano-electronic responses in twisted phosphorene nanoribbons

Many different forms of mechanical and structural deformations have been employed to alter the electronic structure of various modern two-dimensional (2D) nanomaterials. Given the recent interest in the new class of 2D nanomaterials - phosphorene, here we investigate how the rotational strain-dependent electronic properties of low-dimensional phosphorene may be exploited for technological gain. Here, using first-principles density-functional theory, we investigate the mechanical stability of twisted one-dimensional phosphorene nanoribbons (TPNR) by measuring their critical twist angle (θc) and shear modulus as a function of the applied mechanical torque. We find a strong anisotropic, chirality-dependent mechano-electronic response in the hydrogen-passivated TPNRs upon vortical deformation, resulting in a striking difference in the change in the carrier effective mass as a function of torque angle (and thus, the corresponding change in carrier mobility) between the zigzag and armchair directions in these TPNRs. The accompanied tunable band-gap energies for the hydrogen-passivated zigzag TPNRs may then be exploited for various key opto-electronic nanodevices.

Phonon and thermal expansion properties in Weyl semimetals MX (M = Nb, Ta; X = P, As): ab initio studies

Weyl semimetal (WSM) is a new type of topological quantum material for future spintronic devices. Using the first-principles density functional theory, we systematically investigated the thermal expansion properties, and the temperature dependence of isovolume heat capacity and bulk modulus in WSMs MX (M = Nb, Ta; X = P, As). We also presented the phonon dispersion curves and its variation under stress in MX and the anisotropic thermal expansion properties due to the anisotropic crystal structure in WSMs have been predicted in our calculations. Intriguing, we found that the heat capacities increase more rapidly with increasing temperature in the low temperature region for all MX. Furthermore, our results showed that the thermal expansion properties are determined mainly by the isovolume heat capacity at low temperatures, while the bulk modulus has the major effect at high temperatures. These results are useful for applications of WSMs in electronic and spintronic devices.

Ab initio studies of phoshorene island single electron transistor

Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications.

Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues

Phosphorene and phosphorene analogues such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calculation evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temperature nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelectric switching readily readable via electrical, thermal, optical, mechanical, or even spintronic detection upon the swapping of the zigzag and armchair direction. In addition, it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temperature, suggesting high potential for practical device application.

Edge reconstruction in armchair phosphorene nanoribbons revealed by discontinuous Galerkin density functional theory

With the help of our recently developed massively parallel DGDFT (Discontinuous Galerkin Density Functional Theory) methodology, we perform large-scale Kohn-Sham density functional theory calculations on phosphorene nanoribbons with armchair edges (ACPNRs) containing a few thousands to ten thousand atoms. The use of DGDFT allows us to systematically achieve a conventional plane wave basis set type of accuracy, but with a much smaller number (about 15) of adaptive local basis (ALB) functions per atom for this system. The relatively small number of degrees of freedom required to represent the Kohn-Sham Hamiltonian, together with the use of the pole expansion the selected inversion (PEXSI) technique that circumvents the need to diagonalize the Hamiltonian, results in a highly efficient and scalable computational scheme for analyzing the electronic structures of ACPNRs as well as their dynamics. The total wall clock time for calculating the electronic structures of large-scale ACPNRs containing 1080-10,800 atoms is only 10-25 s per self-consistent field (SCF) iteration, with accuracy fully comparable to that obtained from conventional planewave DFT calculations. For the ACPNR system, we observe that the DGDFT methodology can scale to 5000-50,000 processors. We use DGDFT based ab initio molecular dynamics (AIMD) calculations to study the thermodynamic stability of ACPNRs. Our calculations reveal that a 2 × 1 edge reconstruction appears in ACPNRs at room temperature.

Edge-Modified Phosphorene Nanoflake Heterojunctions as Highly Efficient Solar Cells

We propose to use edge-modified phosphorene nanoflakes (PNFs) as donor and acceptor materials for heterojunction solar cells. By using density functional theory based calculations, we show that heterojunctions consisting of hydrogen- and fluorine-passivated PNFs have a number of desired optoelectronic properties that are suitable for use in a solar cell. We explain why these properties hold for these types of heterojunctions. Our calculations also predict that the maximum energy conversion efficiency of these type of heterojunctions, which can be easily fabricated, can be as high as 20%, making them extremely competitive with other types of two-dimensional heterojunctions.

Nonradiative Electron--Hole Recombination Rate Is Greatly Reduced by Defects in Monolayer Black Phosphorus: Ab Initio Time Domain Study

We report ab initio time-domain simulations of nonradiative electron-hole recombination and electronic dephasing in ideal and defect-containing monolayer black phosphorus (MBP). Our calculations predict that the presence of phosphorus divacancy in MBP (MBP-DV) substantially reduces the nonradiative recombination rate, with time scales on the order of 1.57 ns. The luminescence line width in ideal MBP of 150 meV is 2.5 times larger than MBP-DV at room temperature, and is in excellent agreement with experiment. We find that the electron-hole recombination in ideal MBP is driven by the 450 cm(-1) vibrational mode, whereas the recombination in the MBP-DV system is driven by a broad range of vibrational modes. The reduced electron-phonon coupling and increased bandgap in MBP-DV rationalize slower recombination in this material, suggesting that electron-phonon energy losses in MBP can be minimized by creating suitable defects in semiconductor device material.

Anisotropic 2D Materials for Tunable Hyperbolic Plasmonics

Motivated by the recent emergence of a new class of anisotropic 2D materials, we examine their electromagnetic modes and demonstrate that a broad class of the materials can host highly directional hyperbolic plasmons. Their propagation direction can be manipulated on the spot by gate doping, enabling hyperbolic beam reflection, refraction, and bending. The realization of these natural 2D hyperbolic media opens up a new avenue in dynamic control of hyperbolic plasmons not possible in the 3D version.

Mechanical strain effects on black phosphorus nanoresonators

We perform classical molecular dynamics simulations to investigate the effects of mechanical strain on single-layer black phosphorus nanoresonators at different temperatures. We find that the resonant frequency is highly anisotropic in black phosphorus due to its intrinsic puckered configuration, and that the quality factor in the armchair direction is higher than in the zigzag direction at room temperature. The quality factors are also found to be intrinsically larger than those in graphene and MoS2 nanoresonators. The quality factors can be increased by more than a factor of two by applying tensile strain, with uniaxial strain in the armchair direction being the most effective. However, there is an upper bound for the quality factor increase due to nonlinear effects at large strains, after which the quality factor decreases. The tension induced nonlinear effect is stronger along the zigzag direction, resulting in a smaller maximum strain for quality factor enhancement.

Thermal conductivities of single- and multi-layer phosphorene: a molecular dynamics study

As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.

Electronic properties of red and black phosphorous and their potential application as photocatalysts

The promising potential of monolayerrPandbPas photocatalysts was identified, due to their suitable band gap, appropriate band edge position, higher mobility and separation efficiency of charge carriers, and strong response to visible light.

Effect of multilayer structure, stacking order and external electric field on the electrical properties of few-layer boron-phosphide

Development of nanoelectronics requires two-dimensional (2D) systems with both direct-bandgap and tunable electronic properties as they act in response to the external electric field (E-field).

Mechanical and Electrical Anisotropy of Few-Layer Black Phosphorus

We combined reflection difference microscopy, electron transport measurements, and atomic force microscopy to characterize the mechanical and electrical anisotropy of few-layer black phosphorus. We were able to identify the lattice orientations of the two-dimensional material and construct suspended structures aligned with specific crystal axes. The approach allowed us to probe the anisotropic mechanical and electrical properties along each lattice axis in separate measurements. We measured the Young's modulus of few-layer black phosphorus to be 58.6 ± 11.7 and 27.2 ± 4.1 GPa in zigzag and armchair directions. The breaking stress scaled almost linearly with the Young's modulus and was measured to be 4.79 ± 1.43 and 2.31 ± 0.71 GPa in the two directions. We have also observed highly anisotropic transport behavior in black phosphorus and derived the conductance anisotropy to be 63.7%. The test results agreed well with theoretical predictions. Our work provided very valuable experimental data and suggested an effective characterization means for future studies on black phosphorus and anisotropic two-dimensional nanomaterials in general.

Strain induced piezoelectric effect in black phosphorus and MoS2 van der Waals heterostructure

The structural, electronic, transport and optical properties of black phosphorus/MoS₂ (BP/MoS₂) van der Waals (vdw) heterostructure are investigated by using first principles calculations. The band gap of BP/MoS₂ bilayer decreases with the applied normal compressive strain and a semiconductor-to-metal transition is observed when the applied strain is more than 0.85 Å. BP/MoS₂ bilayer also exhibits modulation of its carrier effective mass and carrier concentration by the applied compressive strain, suggesting that mobility engineering and good piezoelectric effect can be realized in BP/MoS₂ heterostructure. Because the type-II band alignment can facilitate the separation of photo-excited electrons and holes, and it can benefit from the great absorption coefficient in ultra-violet region, the BP/MoS₂ shows great potential to be a very efficient ultra-violet photodetector.

Anisotropic Effective Mass, Optical Property, and Enhanced Band Gap in BN/Phosphorene/BN Heterostructures

Phosphorene is receiving great research interests because of its peculiar physical properties. Nonetheless, the phosphorus has a trouble of degradation due to oxidation. Hereby, we propose that the electrical and optical anisotropic properties can be preserved by encapsulating into hexagonal boron nitride (h-BN). We found that the h-BN contributed to enhancing the band gap of the phosphorene layer. Comparing the band gap of the pristine phosphorene layer, the band gap of the phosphorene/BN(1ML) system was enhanced by 0.15 eV. It was further enhanced by 0.31 eV in the BN(1ML)/phosphorene/BN(1ML) trilayer structure. However, the band gap was not further enhanced when we increased the thickness of the h-BN layers even up to 4 MLs. Interestingly, the anisotropic effective mass and optical property were still preserved in BN/phosphorene/BN heterostructures. Overall, we predict that the capping of phosphorene by the h-BN layers can be an excellent solution to protect the intrinsic properties of the phosphorene.

First-Principles Prediction of the Charge Mobility in Black Phosphorus Semiconductor Nanoribbons

We have investigated the electronic structure and carrier mobility of monolayer black phosphorus nanoribbons (BPNRs) using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that the calculated ultrahigh electron mobility can even reach the order of 10(3) to 10(7) cm(2) V(-1) s(-1) at room temperature. Owing to the electron mobility being higher than the hole mobility, armchair and diagonal BPNRs behave like n-type semiconductors. Comparing with the bare BPNRs, the difference between the hole and electronic mobilities can be enhanced in ribbons with the edges terminated by H atoms. Moreover, because the hole mobility is about two orders of magnitude larger than the electron mobility, zigzag BPNRs with H termination behave like p-type semiconductors. Our results indicate that BPNRs can be considered as a new kind of nanomaterial for applications in optoelectronics, nanoelectronic devices owing to the intrinsic band gap and ultrahigh charge mobility.

Graphene-Nanodiamond Heterostructures and their application to High Current Devices

Graphene on hydrogen terminated monolayer nanodiamond heterostructures provides a new way to improve carrier transport characteristics of the graphene, offering up to 60% improvement when compared with similar graphene on SiO2/Si substrates. These heterostructures offers excellent current-carrying abilities whilst offering the prospect of a fast, low cost and easy methodology for device applications. The use of ND monolayers is also a compatible technology for the support of large area graphene films. The nature of the C-H bonds between graphene and H-terminated NDs strongly influences the electronic character of the heterostructure, creating effective charge redistribution within the system. Field effect transistors (FETs) have been fabricated based on this novel herterostructure to demonstrate device characteristics and the potential of this approach.

Magnetic evolution and anomalous Wilson transition in diagonal phosphorene nanoribbons driven by strain

Inducing magnetism in phosphorene nanoribbons (PNRs) is critical for practical applications. However, edge reconstruction and Peierls distortion prevent PNRs from becoming highly magnetized. Using first-principles calculations, we find that relaxed oxygen-saturated diagonal-PNRs (O-d-PNRs) realize stable spin-polarized antiferromagnetic (AFM) coupling, and the magnetism is entirely localized at the saturated edges. The AFM state is quite stable under expansive and limited compressive strain. More importantly, not only does the irreversible Wilson transition occur when applying strain, but the nonmagnetic (NM) metal phase (a new ground state) becomes more stable than the AFM state when the compressive strain exceeds -4%. The related stability and transition mechanism are demonstrated by dual tuning of the geometric and electronic structures, which manifests as a geometric deviation from a honeycomb to an orthorhombic-like structure and formation of P-py bonding (P-pz nonbonding) from P-pz nonbonding (P-py antibonding) because of the increase of the proportion of the P-py (P-pz) orbital.

Phosphorene: Fabrication, Properties, and Applications

Phosphorene, the single- or few-layer form of black phosphorus, was recently rediscovered as a two-dimensional layered material holding great promise for applications in electronics and optoelectronics. Research into its fundamental properties and device applications has since seen exponential growth. In this Perspective, we review recent progress in phosphorene research, touching upon topics on fabrication, properties, and applications; we also discuss challenges and future research directions. We highlight the intrinsically anisotropic electronic, transport, optoelectronic, thermoelectric, and mechanical properties of phosphorene resulting from its puckered structure in contrast to those of graphene and transition-metal dichalcogenides. The facile fabrication and novel properties of phosphorene have inspired design and demonstration of new nanodevices; however, further progress hinges on resolutions to technical obstructions like surface degradation effects and nonscalable fabrication techniques. We also briefly describe the latest developments of more sophisticated design concepts and implementation schemes that address some of the challenges in phosphorene research. It is expected that this fascinating material will continue to offer tremendous opportunities for research and development for the foreseeable future.